Poly(methyl methacrylate) (PMMA) is very famous for its optic isotropy. It shows little birefringence with flow or deformation during processing. Thus, PMMA is widely used for optical apparatus. Nowadays, higher optical isotropy is required for replacement of glass components in portable devices. However, little work has done about birefringence or stress-optical coefficient (SOC) of PMMA because it was very small and there is a difficulty in accuracy of birefringence measurements. In this study, SOC of PMMA films during uniaxial stretching was measured in various temperatures above glass transition temperature (Tg) of PMMA. In addition, temperature dependence of birefringence of oriented PMMA films was measured below Tg. SOC of PMMA was decreased with increasing stretching temperature. It was -35 TPa-1 at Tg+10 deg. (130 deg. C.), on the other hand, it was only -11 TPa-1 at Tg+30 deg. (150 deg. C.). These results was far from the theory based on entropic elasticity caused by molecular orientation. To understanding more deeply in birefringence of PMMA, temperature dependence of birefringence of oriented PMMA films was measured below Tg. Absolute value of birefringence of oriented PMMA film was decreased with increasing temperature. In addition, change of birefringence was reversible in heating and cooling below Tg. However, once temperature exceeded Tg, irreversible decrement was observed with thermal shrinkage. It suggested that the most stable form of side chains was varied with temperature even below Tg and it affected on birefringence of PMMA films. Temperature dependence of SOC has similar tendency with temperature dependence of birefringence. It means that SOC of PMMA was also affected by the conformation of side chains.